Generally, a rise in the temperature of a pneumatic tire promotes a time-dependent change in a material constituting the pneumatic tire. An example of the time-dependent change in the material is a change in properties. The temperature rise in the pneumatic tire can also cause damage in a tread portion at high-speed driving. For those reasons, the temperature rise in the pneumatic tire is not desirable in terms of the durability of the pneumatic tire.
An improvement in the durability is important particularly for an off-the-road radial tire (ORR) and a truck and bus radial tire (TBR) that receive a heavy load, as well as a run-flat tire running flat (running with a tire internal pressure of 0 kPa). Accordingly, it is important for these pneumatic tires to have a reduced temperature rise.
For example, the run-flat tire is provided with sidewall reinforcing layers for reinforcing tire side portions. The sidewall reinforcing layers each have a crescent shape in a cross section taken in a tire width direction. When the run-flat tire runs flat, deformation in a tire radial direction concentrates on the sidewall reinforcing layers. Thereby, the sidewall reinforcing layers have an increased temperature, consequently deteriorating the durability of the run-flat tire.
On the other hand, there has been proposed a technique in which reinforcing members for suppressing deformation of the pneumatic tire are provided to a carcass layer and bead portions (called a first technique below) (e.g., Patent Literature 1). In the first technique, the reinforcing members for suppressing deformation in side portions are provided particularly to the carcass layer and the bead portions that constitute the tire side portions. Thereby, the temperature rise in the pneumatic tire, or particularly, the temperature rise in the tire side portions can be suppressed.
There is known a technique in which a rim guard having many ridges is provided outside, in the tire width direction, of each bead portion adjacent to a rim (called a second technique below). The surface area of the pneumatic tire is increased by the many ridges provided to the rum guard, improving the radiation performance of heat generated in the pneumatic tire.